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Microstructure and Some Magnetic Properties of Nanocrystalline Fe73.5-xCoxCu1Nb3Si13.5B9 Alloys
Józef Zbroszczyk
To cite this version:
Józef Zbroszczyk. Microstructure and Some Magnetic Properties of Nanocrystalline Fe73.5- xCoxCu1Nb3Si13.5B9 Alloys. Journal de Physique I, EDP Sciences, 1997, 7 (8), pp.1019-1030.
�10.1051/jp1:1997195�. �jpa-00247375�
Microstructure and Some Magnetic Properties of Nanocrystalline Fe73_5-~Co~CuiNb3Si135Bg Alloys
J6zef Zbroszczyk (*)
Institute of Physics, Technical University of Cz~stochowa, Cz~stochowa, Poland
(Received 13 August 1996, revised 28 January1997, accepted 18 April 1997)
PACS.75.60.Lr Magnetic aftereffects
PACS.76.80.+y M6ssbauer effect; other y-ray spectroscopy
PACS.75.50.Kj Amorphous and nanocrystalline magnetic materials; quasicrystals
Abstract. The microstructure and magnetic properties, I,e. the initial susceptibility, co-
ercivity and saturation magnetic polarization of nanocrystalline Fe73.5-~CoxCuiNb3Sii3.5Bi (x = 0 or 7) alloys with different volume fractions of crystalline and amorphous phases, are in-
vestigated. The crystalline phase in both conventionally annealed alloys (823 K for 1h) has D03 ordered structure. However, after the accumulative annealing of the samples (823 K for 5 s and then 10 and 60 min) only short range order in the crystalline phase is observed. The replacing of
7it Fe atoms by Co atoms in the Fe73.5CuiNb3Si13.5Bg alloy leads to the slight enhancement of
the magnetic saturation polarization for both as-quenched and nanocrystalline samples. More- over, the initial susceptibility distinctly increases and disaccommodation as well as coercivity decrease with annealing time for both investigated alloys. This behaviour is attributed to the annealing out of &ee volumes in the amorphous matrix and the increase of the volume fraction of the crystalline phase. Furthermore, the results reported in this paper indicate that processes occurring in the amorphous matrix are the main source of the magnetic after-effect in these alloys.
1. Introduction
Nanocrystalline Fe-Cu-Nb-Si-B alloys obtained by controlled crystallization of amorphous rib- bons ionsist of
a crystalline a-Fesi phase and an amorphous matrix [1-6]. The combined ad- dition of Cu and Nb into Fe-Si-B amorphous alloys allows to obtain a nanostructured material with a mean grain size of the order of10 nm [1, 2, 5,6] after annealing the amorphous ribbons at about 823 K for 1 h. The ultrafine grain structure of these alloys leads to the excellent soft magnetic properties which arise from the great reduction in the mean magnetocrystalline anisotropy and very low saturation magnetostriction [6]. The soft magnetic properties (e.g.
coercivity, initial susceptibility) of the nanocrystalline alloys are highly influenced by their
phase composition which results from annealing conditions [2]. The volume fraction of the amorphous and crystalline phases in these materials can be determined from X-ray diflrac- tometry [2) or M6ssbauer spectroscopy studies [7]. However, local changes in the structure of these materials may be also detected by investigations of the magnetic after-effect [8) which
(*) e-mail: jozektlmim.pcz.czest.pl
© Les iditions de Physique 1997
is usually observed as a disaccommodation I.e. temporal decrease of the initial susceptibility after the sample demagnetization. The investigations of the disaccommodation also allow the
contribution of the amorphous and crystalline phases to this effect to be found.
The Fe73.5CuiNb3Si13.589 alloy is a typical example of the nanocrystalline alloys and has a promising application because of its high initial permeability and saturation flux density [1,6,9].
The aim of this paper is to investigate the microstructure, initial magnetic susceptibility, disaccommodation, saturation magnetic polarization and coercivity for the nanocrystalline Fe73.5-~Co~CuiNb3Si13.589 (x
= 0 or 7) alloys containing a different quantity of crystalline phases.
2. Experimental Procedure
The amorphous Fe73.5-~Co~CuiNb3Si13.589 (x
= 0 or 7) ribbons were obtained by a melt-
quenching technique. The width and thickness of the ribbons were 10 mm and 20 pm, respec-
tively.
The microstructures of the samples were examined by M6ssbauer spectroscopy and X-ray diffractometry. The M6ssbauer spectra were fitted with a superposition of seven components:
five sextets for the crystalline phase and two sextets for the amorphous matrix (corresponding
to low- and high-field components). From the numerical analysis of these M6ssbauer spectra the magnetization distribution, average hyperfine field, content of iron in the crystalline and
amorphous phases, their volume fraction and order degree in the crystalline phase were deter- mined. The average hyperfine field was evaluated from the hyperfine field distribution obtained
according to the Hesse-Riibartsch method [10].
In order to determine the iron content jam in the amorphous phase, we assume that during
the crystallization of the samples no significant structural changes occur in the amorphous
matrix (as compared to the as-quenched state). However, a decrease of iron or iron and cobalt content takes place. The assumption that average hyperfine field is proportional to the iron (or
iron and cobalt) content in the amorphous phase [11,12] seems to be a good approximation.
Thus, jam for the Fe73.5CuiNb3Si13.589 alloy was calculated from equations:
lam ~~')~~~~~~~~(l~am) (l)
It is known that the volume fraction of a phase is proportional to the relative area of the M6ssbauer spectrum. Taking into account a change of the iron content in the amorphous
matrix and equation (1), the volume fraction ((m) of the amorphous phase may be estimated from:
~ ~~~~fll,~8~~
(~)
am jf
am
where R is the relative area of subspectra corresponding to the amorphous matrix obtained from the M6ssbauer spectra analysis, (Ham,asq) and (Ham) are the average hyperfine fields
obtained for the amorphous as-quenched sample and amorphous matrix, respectively. Using
the resultb obtained for the amorphous matrix it is possible to calculate the volume fraction of the crystalline phase and its composition. The volume fractions of the amorphous and
crystalline phases and their compositions in the Fess.5Co? CuiNb3Sii3.589 alloy were estimated in a similar way. In these calculations we assumed the same probability of recoil free absorption
for the amorphous matrix and crystalline phase.
Taking into account values of hyperfine fields and isomer shift, five sextets corresponding to the crystalline o-Fesi or a-(Fe,Co)Si phases were ascribed to the iron atoms having 8, 7, 6,
5 and 4 iron or iron and cobalt atoms as nearest neighbours. The probabilities (P~) for iron
atoms to have I iron atoms as nearest neighbours were determined from:
~i " /~ (3)
~ Ri
i=4
where R~ are the relative areas of the corresponding sextets. Knowing values of these proba- bilities one may calculate the average number of Si atoms in the first coordination zone of iron
atoms using the formula:
8
n = 8 ~j Pi I. (4)
1=4
The short range order parameter (al was calculated using the formula for binary Fe-Si alloys [13] from the equation:
al = ~ o~~ (5)
where no is the average number of Si atoms in the neighbourhood of Fe atoms arranged randomly on a bcc lattice obtained from a binomial distribution, and n is calculated from equation (4). The long range order parameter 16'/) for the D03 structure is defined as:
N
i = j
where No is the total number of Si sites in the D03 superstructure, N is the number of Si atoms occupying Si sites. This parameter 16'/) was calculated [13] from the following expression:
~~'~"~~ ~ ~~ ~~ '~~
~~~ ~~~.7/~~ ~
~
~ ~~.7/~ ~ ~
~~~~~ 'lls~~ ~ '~ lls~~ ~ ~ ~~ ~ ~~~.7/~~~
~
~
~ ~~.7/~~~ ~~~
~~~
0.25 cl
~~ c(1 -1) " c(1 1) ~~"
~ 1-c ~ 0.75 ~ 0.75
where P(n,i) are the probabilities for iron atoms to have n Si atoms as nearest-neighbours,
and c is silicon concentration in the crystalline a-Fesi and a-(Feco)Si phases.
Using a completely automated set-up, the magnetic susceptibility for toroidal samples of 30 mm inner diameter was measured within the temperature range from 250 up to 650 K.
For the same samples the disaccomodation of the initial susceptibility was investigated. Be-
fore each measurement the samples were demagnetized by the sinusoidal magnetic field of
frequency 120 Hz with exponentially decreasing amplitude from 500 A/m to zero during 1.1 s.
The investigations were carried out in an ac magnetizing field of frequency f = 2 kHz and amplitude H
= 0.16 A/m. The experimental results are presented as isochronal curves which
are constructed such that:
A(I/x)
= 1/x2 -1/xi (7)
where 1/xi and 1/x2 are reciprocal magnetic susceptibilities at times ti = 2 s and t2 " 120 s after demagnetization, respectively. The coercive field was measured by a F6rster coercimeter
c
b
m~ ~,~ ~
~
m
~
Q
~00 600 700
T(K) T(K)
Fig. 1 Fig. 2
Fig. 1. Temperature dependence of the initial magnetic susceptibility x (measured 2 s after de-
magnetization) for the Fe73.5CuiNb3Si13.5Bg alloy after accumulative annealing at 823 K for: (a) 5 s, (b) 10 min, (c) 60 min.
Fig. 2. The initial magnetic susceptibility x (measured 2 s after demagnetization) versus temper- ature for the Fess.5Co7CuiNb3Si13.5Bg alloy after accumulative annealing at 823 K for: (a) 5 s, (b)
10 min, (c) 60 min.
for 10 cm long strips. All investigations were performed for the samples annealed at 823 K for 5 s, 10 min and 60 min (accumulative annealing) and for the ribbons treated at 823 K for 1 h
(conventional annealing).
The saturation magnetic polarization was measured by a force magnetometer taking Samples
in the form often discs of 3.5 mm diameter cut out from ribbons and arranged along the rolling direction. These measurements were performed in the temperature range from 300 to 770 K for the samples in the as-quenched state and after the last step of the accumulative annealing (at 823 K for1 h).
Additionally, for the as-quenched samples the saturation magnetostriction was measured by the transverse susceptibility method [14].
3. Results
The initial magnetic susceptibility x (measured 2 s after the demagnetization) versus temper-
ature for the Fe73.5CuiNb3Si13.589 alloy after the accumulative annealing of the amorphous
ribbons at 823 K. for 5 s and then 10 min and 60 min is presented in Figure 1. As can be
seen from this figure, the initial susceptibility reaches a maximum at temperatures between 450 to 570 K and then rapidly decreases with temperature. Moreover, the maximum value of the initial susceptibility distinctly increases with the annealing time.
Figure 2 shows the corresponding results of the initial susceptibility measurements for the
sample of the Fess.5Co7CuiNb3Si13.5Bg alloy after the same heat treatments as the previ-
ous one. The initial susceptibility of this sample runs through a maximum at tempera-
tures between 350 and 550 K but x(T) curves are quite different from those obtained for
the Fe73.5CUlNb3Sl13.589 Sample.
$ ~
G ~
~ ~
o o
a
300 (00 500 600 700
TlKl T(K)
Fig. 3 Fig. 4
Fig. 3. After-effect spectra for the Fe73.5CuiNb3Si13.5Bg alloy after the accumulative treatment at 823 K for: (al 5 s, 16) 10 min, (c) 60 min.
Fig. 4. Isochronal relaxation spectra of the Fe66.5Co7CuiNb3Si13.5Bg alloy after the accumulative
annealing at 823 K for: 5 s la), 10 min (b), and (c) 60 min.
The isochronal disaccommodation curves A(1/x)
= f(T) obtained from the initial suscepti- bility x measurements at 2 and 120 s after the demagnetization (Eq. (7)) for the
Fe73.5CuiNb3 Si13.589 ribbon after the accumulative heat treatment are shown in Figure 3.
The sample annealed at 823 K for 5 s shows a very broad relaxation maximum at about 500 K.
Above 570 K the intensity of the disaccommodation rapidly increases. After the subsequent annealing of this sample at 823 K for 10 and 60 min. no relaxation processes are observed
between 250 and 500 K except for an almost temperature independent background. Above 550 K the disaccommodation intensity distinctly increases.
Figure 4 shows the isochronal disaccommodation curves (constructed according to Eq. (7)) of the Fess.5Co7CuiNb3Si13.589 alloy after the same treatments as for the Fe73.5CuiNb3Si13.589 alloy. It is seen that the curves A(1/x) = f(T) show similar shapes as those obtained for the
Fe73.5CuiNb3Si13.589 alloy. However, the very broad maximum in A(1/x)
= f(T) curve for
the former alloy annealed at 823 K for 5 s is shifted towards the higher temperature.
The temperature dependence of the initial susceptibility and the isochronal A(1/x)
= f(T)
curves for the Fe73.5CuiNb3Si13.589 and Fess.5Co7CuiNb3Si13.589 alloys annealed for 823 K
for 1 h (conventional annealing) are shown in Figures 5 and 6. It is seen that the initial susceptibility for the Fe73.5CuiNb3Si13.589 alloy depends to some degree on temperature up to 500 K. At higher temperatures x distinctly decreases. However, the initial susceptibility of the Fess.5Co7CuiNb3Si13.589 alloy decreases slowly in the whole temperature range (from
250 to 650 K). Moreover, the disaccommodation amplitude for both samples increases rapidly
above 500 K.
The magnetization curves poms
= f(T) for the Fe73.5-~Co~CuiNb3Si13.589 (~
= 0 or
7) alloys in the as-quenched state and after the last step of the accumulative annealing (823 K for h), as an example, are shown in Figure 7. The magnetic saturation polariza-
tion poms decreases monotonically with temperature for both investigated samples. Moreover,
a
b a
$
~ ~
° ~ /
o o
550 T(Kl
Fig. 5 Fig. 6
Fig. 5. The initial magnetic susceptibility x (measured 2 s after demagnetization) versus temper-
ature (a) and the isochronal All /x) = f(T) curve (b) for the Fe73.5CuiNb3Si13.5Bg alloy after the
conventional annealing at 823 K for h.
Fig. 6. The initial magnetic susceptibility x (measured 2 s after demagnetization) uersus tempera- ture (a) and the isochronal All /x)
= f(T) curve (b) for the Fe66.5Co7CuiNb3Si13 5Bg alloy after the
conventional annealing at 823 K for 1 h.
the saturation polarization slightly increases after replacing 7il Fe atoms by Co atoms in the Fe73.5CuiNb3Si13.589 alloy.
In Figure 8 the M0ssbauer spectra for the as-quenched and annealed at 823 K for 1 h (the
last step of the accumulative annealing) samples are presented as an example. The results obtained from M0ssbauer spectra analysis are shown in Table I. In this table the coercivity for the as-quenched and annealed samples, and saturation magnetostriction for the as-quenched
ribbons are also shown.
4. Discussion
M0ssbauer spectroscopy and X-ray diffractometry investigations show that the Fe73.5-~ Co~cui Nb3Si13.589 (x
= 0 or 7) alloys are fully amorphous in the as-quenched state. After the
replacement of 7$~ Fe atoms by Co atoms in the Fe73.5CuiNb3Si13.589 alloy, the enhancement of the average hyperfine field at ~~Fe nuclei (Tab. I) is observed. This is in agreement with
the results obtained from measurements of magnetization curves (Fig. 7); the addition of Co atoms slightly increases the magnetic saturation polarization by comparison with poms for the Fe73.5CuiNb3Si13.589 alloy. Similar results were obtained by Guo et al. [15] from M6ssbauer
studies of amorphous Fe-Cc-Si-B alloys. The increase of the saturation magnetization is also observed for crystalline Fe-Co alloys up to about 30$~ of Co [16].
From the changes of the ratios of the line intensities in the Zeeman sextets (3: (A2,5)1 1) (Tab. I), it may be concluded that the tendency for the magnetization vector to align in
parallel to the ribbon surface increases after annealing the investigated samples at 823 K for 5 s as compared to the as-quenched state. This is connected with the stress-relief of the ribbons
during the treatment. However, the intensity of the second line in the Zeeman sextets decreases
a
~
b
o
g I E
~
C f~
Z
d ~
QJ W
~ d
~~
~o
a
'00 500 600 700 800 8
T K Velocity mm Is
Fig. 7 Fig. 8
Fig. 7. -Temperature dependence of the magnetic saturation polarization poms for the Fe73.5CuiNb3Si13.5Bg alloy: (a) as-quenched state, (b) conventionally annealed at 823 K for h and for the Fess 5Co7CuiNb3Si13.5Bg alloy: (c) as-quenched state, (d) conventionally annealed at 823 K for I h.
Fig. 8. M6ssbauer spectra for the Fe73.5CuiNb3Si13.5Bg and Fe66.5Co7CuiNb3Si13.5Bg alloys:
la, c) as-quenched state, 16, d) after the last step of accumulative annealing at 823 K (60 min), respectively.
after the subsequent heat treatments of these samples at 823 K for 10 and 60 min (Tab. I).
This indicates that the accumulative annealing influences the magnetization distribution in the
investigated samples [17].
The iron or iron and cobalt contents in both the crystalline and amorphous phases depends
on the annealing time (Tab. I). It is known [18,19] that the a-Fesi grains grow from the nuclei which are formed in the iron-rich regions due to the diffusion of Cu, Nb and B atoms from these regions into the surrounding amorphous phase. When the annealing time is short, the diffusion of these atoms may be incomplete. With increasing time of the heat treatment more and more of the Cu, Nb and B atoms diffuse into the amorphous matrix and the iron content in the crystalline phase increases (Tab. I). Moreover, the amorphous phase is enriched with Nb, Cu and B. It is worth noticing that average hyperfine fields of the amorphous matrix are equal
to 16.6 and 16.7 T for the Fe73.5CuiNb3Si13.5Bg and Fess.5Co7CuiNb3Si13.589 alloys (after
accumulative annealing at 823 K for 1 h), respectively. However, the average hyperfine field for the nanocrystalline Fess.5Co7CuiNb3Si13.589 alloy is higher than for the Fe73.5CuiNb3Si13,5Bg
Table I. The effective hyperfine field at ~~Fe nuclei (Bejj ), intensity of the second line in Zeeman sextets (A2), uolume fraction of the amoYphous and crystalline phases ~vam, Vcr),
iron content in the amorphous and crystalline phases (fain, for), iron and cobalt content in
the amorphous and crystalline phases (gain, gcr), short and long range order parameters (al, i), coerciuity (Hc ) and saturation magnetostriction constant (ls) obtained for the as-quenched
and annealed Fe7s.5CuiNbsSii3.589 and Fess.5Co7CuiNb3Si13.589 alloys.
Fe73 5CuiNb3Si13.5Bg
treatment A2 irr Vcr fart for al 7 As x 10~ Hc (A/m)
accumulative annealing
as-q 20.58 2.5 1 0 73.5 1.94 13.3
823 K/5 s 22.97 3.9 0.7 0.3 72.9 75 0.17 3.8
823 K/10 min 23.01 3.8 0.6 0.4 70.5 78 0.23 2.6
823 K/1 h 23.18 3.7 0.48 0.52 66.3 80 0.46 2.0
conventional annealing
823 K/1 h 23.22 3.7 0.49 0.51 64 82 0.85 1.8
Fe66.5Co7CuiNb3Si13.5Bg
treatment BeA[Tj 2i2 Varr Vcr 9am 9cr "1 'l ~s X 10~ Hc
accumulative annealing
as-q 21.18 2.3 0 73.5 2.16 10.1
823 K/5 s 23.58 3.2 0.61 0.39 70.1 78.9 0.25 3.4
823 K/10 min 23.77 3.0 0.54 0.46 68 80 0.35 2.5
823 K/1 h 23.70 2.7 0.47 0.53 64.7 81.3 0.7 2.1
conventional annealing
823 h 23.70 3.9 0.45 0.55 60 83 0.75 IA
one (Tab. I). From these results it is possible to conclude that during crystallization of the
Fess_5Co7CuiNb3Si13.5Bg alloy a crystalline (Feco)-Si phase is formed.
From M6ssbauer spectra analysis it was found (Tabs. II and III) that the probabilities (equal
to relative areas of corresponding sextets) for iron atoms to have 8, 7,6, 5 or 4 Fe atoms as the nearest neighbours (nn) in the crystalline a-Fesi phase (in the Fe73.5CuiNb3Si13.5Bg alloy) are
different from those obtained from binomial distribution. It is seen that the probabilities of
occurrence of5 and 4 Fe atoms as nn are larger that those in the random model (binomial distri-
bution) (Tab. II). Furthermore, the probability for iron atoms to have 8 Fe atoms as the nearest
neighbours is much lower than it should be for the D03 structure [13], Eq. (6)). Thus, we
may conclude that in the crystalline a-Fesi phase (after accumulative annealing of the sample
at 823 K for 5 s, 10 and 60 min) only the short range order occurs (Tabs. I and II). Moreover,
one can notice that the a-Fesi phase becomes more ordered with the increase of annealing
time (Tab. I). Similar effect is observed for the nanocrystalline Fess.5 Co? CuiNb3Sii3.5Bg alloy obtained by the accumulative annealing the amorphous ribbons (Tabs. I and III). However, the results obtained from M6ssbauer spectra analysis for the conventionally annealed samples of the Fe73_5CuiNb3Si13.5Bg and Fess.5Co7CuiNb3Si13.5Bg alloys (823 K for h) are similar to those obtained for D03 structure (Eq. (6) and Tabs. II and III). This indicates that in the
crystalline phase of conventionally annealed alloys, the long range order of D03-type is present (Tab. 1).
